Fix circuit drawer returning qubit[register] names for input (#11096)

* added registers to layout in sabre_layout pass

* Add reno and test

* Fix layout method in test

* Set transpiler seed

* modified reference circuit for test

---------

Co-authored-by: Elena Peña Tapia <epenatap@gmail.com>
Co-authored-by: Elena Peña Tapia <57907331+ElePT@users.noreply.github.com>

qiskit/transpiler/passes/layout/sabre_layout.py

@@ -279,6 +279,10 @@ class SabreLayout(TransformationPass):
             }
         )
 
+        # Add the existing registers to the layout
+        for qreg in dag.qregs.values():
+            self.property_set["layout"].add_register(qreg)
+
         # If skip_routing is set then return the layout in the property set
         # and throwaway the extra work we did to compute the swap map.
         # We also skip routing here if there is more than one connected

releasenotes/notes/fix-qreg-visualization-after-layout-42d3e643b923d8bc.yaml

@@ -0,0 +1,8 @@
+---
+fixes:
+  - |
+    Fixed a bug in :class:`~.SabreLayout` where it would fail to add the layout
+    register information to the property set. This affected circuit visualization, as
+    ``circuit.draw()`` after transpilation with certain optimization levels would show
+    the full ``Qubit[register]`` label rather than the expected register name
+    (e.g. ``q0``).

test/visual/mpl/circuit/test_circuit_matplotlib_drawer.py

@@ -1,6 +1,6 @@
 # This code is part of Qiskit.
 #
-# (C) Copyright IBM 2020.
+# (C) Copyright IBM 2020, 2023.
 #
 # This code is licensed under the Apache License, Version 2.0. You may
 # obtain a copy of this license in the LICENSE.txt file in the root directory
@@ -2177,6 +2177,7 @@ class TestCircuitMatplotlibDrawer(QiskitTestCase):
         backend.target.add_instruction(SwitchCaseOp, name="switch_case")
         backend.target.add_instruction(IfElseOp, name="if_else")
         tqc = transpile(qc, backend, optimization_level=2, seed_transpiler=671_42)
+
         fname = "nested_layout_control_flow.png"
         self.circuit_drawer(tqc, output="mpl", filename=fname)
 
@@ -2208,6 +2209,31 @@ class TestCircuitMatplotlibDrawer(QiskitTestCase):
             ):
                 qc.draw("mpl", style=style)
 
+    def test_no_qreg_names_after_layout(self):
+        """Test that full register names are not shown after transpilation.
+        See https://github.com/Qiskit/qiskit-terra/issues/11038"""
+        backend = FakeBelemV2()
+
+        qc = QuantumCircuit(3)
+        qc.cx(0, 1)
+        qc.cx(1, 2)
+        qc.cx(2, 0)
+        circuit = transpile(
+            qc, backend, basis_gates=["rz", "sx", "cx"], layout_method="sabre", seed_transpiler=42
+        )
+
+        fname = "qreg_names_after_layout.png"
+        self.circuit_drawer(circuit, output="mpl", filename=fname)
+
+        ratio = VisualTestUtilities._save_diff(
+            self._image_path(fname),
+            self._reference_path(fname),
+            fname,
+            FAILURE_DIFF_DIR,
+            FAILURE_PREFIX,
+        )
+        self.assertGreaterEqual(ratio, 0.9999)
+
 
 if __name__ == "__main__":
     unittest.main(verbosity=1)